Rotary cutting tools

ABSTRACT

A rotary cutting tool having a tool body insertable into a drill string with a plurality of projecting or extendable cutter assemblies at positions distributed azimuthally around the longitudinal axis of the tool body. Each cutter assembly includes a support structure bearing a plurality of cutters with exposed hard cutting surfaces facing in a direction of rotation of the tool. The support structure may have a surface positioned to contact a conduit wall and may be made as a body of particulate material infiltrated with a metal binder, where the particulate material may include material such as tungsten carbide having a Knoop hardness of at least 1000. The support structure may have a functionally graded composition with hard material to contact the wall and steel in a region which will be machined during manufacture.

CROSS-REFERENCE TO RELATED APPLICATION

The present document is based on and claims priority to U.K. Non-Provisional Application Serial No. 1601130.6, filed Jan. 21, 2016, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure is concerned with a rotary cutting tool for incorporation into a drill string to enlarge an underground conduit. This conduit may be a borehole drilled through geological formation(s) and the tool may be a reamer whose purpose is to enlarge the diameter of such a borehole. The conduit may alternatively be the interior of tubing placed within a borehole and the tool may operate to remove material from the interior of the tubing, possibly to the extent of entirely destroying a section of tubing.

BACKGROUND

It is normal practice that a rotary cutting tool such as a reamer can be incorporated in a drill string extending from surface or alternatively attached to coiled tubing extending from the surface. Drilling fluid is pumped down the drilling string or coiled tubing to the reamer tool and returns to the surface outside tubing with cuttings entrained in the returning fluid.

The purpose of a reamer is to enlarge a borehole which already exists. Consequently it is characteristic that a reamer has a tool body with a longitudinal axis and connectors at the upper and lower ends for attaching to the drill string all cutters spaced radially outwardly from the axis of the tool an axially leading part of the tool, which may be a connector for attaching to part of a drill string, positioned axially ahead of all cutters.

A reamer may have a plurality of cutter assemblies, each comprising a support structure with attached cutters, arranged azimuthally around the axis of the tool. An illustration of a cutter assembly which is integral with the tool body and projects from it is seen in U.S. Pat. No. 6,386,302. Some reamers are expandable, which allows the tool to be inserted through tubing and then expanded and pushed axially forward within open hole below the tubing, in order to increase the diameter of the open hole. In the case of an expandable reaming tool there may be a plurality of such cutter assemblies which extend axially and are expandable radially outwardly. A mechanism for expanding these cutter assemblies radially outwardly from the axis may use hydraulic pressure from the drilling fluid to force the support structures of the cutter assemblies outwardly. Expandable reamers are illustrated by various documents including U.S. Pat. Nos. 6,732,817 and 7,954,564.

Cutter assemblies for a reamer have conventionally been made with a steel casting as the support structure. Steel can readily be machined to required dimensions after casting. Cutters attached to the supporting structure may be hard faced and may be so-called PDC cutters having a body with a polycrystalline diamond section at one end. The body may be made from hard material such as sintered tungsten carbide particles. The polycrystalline diamond section which provides the cutting part may then comprise particles of diamond and a binder. In many instances, the polycrystalline diamond section is a disc and so the hardest end of a cutter is a flat surface. However, the polycrystalline diamond section but can also have other shapes.

As shown by U.S. Pat. Nos. 6,732,817 and 7,954,564, reamer designs customarily position at least some cutters with their cutting faces at the leading face of a support structure and with the cutters projecting radially outwardly beyond the support structure. The parts of the cutter which project outwardly beyond the support structure may be the parts of the cutter principally involved in cutting as the rotating reamer is advanced axially and/or as an expandable reamer is expanded radially in preparation for axial advance.

The greatest radius swept by a reamer (so-called full gauge) may be the radial distance from the axis to the extremity of the outermost cutter(s). In order to position a reamer centrally in the reamed borehole, it is customary for a supporting structure to include a section which does not include cutters but has a so-called gauge pad (alternatively spelt “gage pad”) which is a surface positioned to confront and slide on the wall of the reamed borehole. In an expandable reamer, it is common practice to position gauge pads at a radius which is slightly less than full gauge so as to facilitate cutting radially outwardly during the period when the reamer is being expanded.

SUMMARY

This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to be used as an aid in limiting the scope of the claimed subject matter.

One aspect of the present disclosure relates to a rotary cutting tool to enlarge an underground conduit, comprising a tool body and one or more cutter assemblies attached to the tool body wherein each cutter assembly includes a support structure to which a plurality of cutters are fitted with leading surfaces facing in a direction of rotation of the tool. The support structure, to which at least one hard surfaced cutter is attached, includes at least one surface which is positioned to contact the conduit wall. In some forms of rotary tool, such a surface may face radially outwardly and be aligned with the radial extremity of at least one cutter such that the surface contacts the conduit wall after the cutter has travelled across the wall. Alignment of the surface with the radial extremity of an exposed part of a cutter may be such that the cutter does not project outwardly beyond the said outward-facing surface. Because the tool is for enlarging a conduit which has already been created, it may have connectors at the upper and lower ends of the tool body for attaching to a drill string and/or may be constructed with all cutters axially behind the lower (and therefore axially leading) end of the tool body.

As taught in currently unpublished GB patent applications, such a surface which contacts the conduit wall traversed by a cutter extremity can be beneficial in positioning the rotary tool relative to the conduit which is being enlarged and reducing vibration. However, the surface is subject to wear when it makes sliding contact with the conduit wall.

In this aspect of the present disclosure, the support structure bounded by the said surface which is positioned to contact the conduit wall comprises a body of particulate material infiltrated with a metal binder and the particulate material includes material having a Knoop hardness of at least 1000, possibly at least 1300 or at least 1600.

A Knoop hardness greater than 1000 differentiates hard materials, for example tungsten carbide, from steel. Tungsten carbide has a reported hardness of 1880 on the Knoop scale whereas tool steel has been reported to have a hardness of approximately 770 and some other grades of steel have a hardness below 500 on the Knoop scale.

In this disclosure the term “hard material” will be used to mean material with Knoop hardness of least 1000.

At least at the support structure's surface which is positioned to contact the conduit wall, the particulate material may consist of 30 to 100% of hard material possibly from 50% to 100% of hard material while the balance (if any) of particulate material with Knoop hardness less than 1000 may be steel, alloy steel or other metal.

The particulate material in a support structure may be of uniform composition, that is to say having the same composition throughout the body, in which case the particulate material may be a single hard material or it may be a mixture of particulate materials at least some of which are hard material. However, the particulate material may be a so-called functionally graded material composition which changes from one region of the support structure to another. A region bounded by the surface which is positioned to contact the conduit wall may have a higher proportion of hard material than a region which is spaced away from that surface. So the particulate material in the region bounded by the surface which is positioned to contact the conduit wall (and which may be a radially outward facing surface) may comprise from 30, 40 or 50% up to 100% of hard material while the region spaced away from that surface (which may be a region through which the support structure is attached to the tool body) may comprise no more than 25% hard material while one or more regions in between have an intermediate composition so as to provide a progressive transition in composition between the two spaced apart regions. As the concentration of hard particles increases, the concentration of particles with Knoop hardness below 1000, which may be steel particles, decreases. The particulate material may be from 30, 40 or 50% up to 100% of hard material with a Knoop hardness of at least 1300 in the region bounded by the surface which contacts the conduit wall and at least 75% of material with Knoop hardness below 800 in the region spaced away from that surface, all of these percentages being by weight.

A benefit of such a functionally graded material is that it provides hard material to resist wear of the surface which contacts the conduit wall while facilitating machining of the other region after infiltration has taken place.

Thus a second aspect of the present disclosure can be stated as a rotary cutting tool for incorporation into a drill string to enlarge an underground conduit, comprising a tool body having a longitudinal axis extending between upper and lower ends with connectors at the upper and lower ends for attaching to the drill string and further comprising a plurality of cutter assemblies attached to the tool body at positions distributed azimuthally around the longitudinal axis of the tool, wherein each cutter assembly comprises a supporting structure bearing a plurality of cutters with exposed hard cutting surfaces providing the cutters' leading surfaces in the direction of rotation of the tool, and wherein the supporting structure is a body of particulate material infiltrated with a metal binder, the composition of the particulate material varying progressively from a first region of the support structure which is bounded by a surface positioned to contact the conduit wall to a second region which is spaced from the first said region (and which may be attached to the tool body) such that the first region contains particulate material with Knoop hardness of at least 1000 at a greater concentration than the second region. The surface positioned to contact the conduit wall may face radially outwardly.

Another possibility, which is a further aspect of the present disclosure, is that the process of forming a body of infiltrated particulate material can be used to embed or partially embed objects which are larger than the particles of the particulate material. This aspect of the present disclosure provides a rotary cutting tool for incorporation into a drill string to enlarge an underground conduit, comprising a tool body having a longitudinal axis extending between upper and lower ends with connectors at the upper and lower ends for attaching to the drill string and further comprising a plurality of cutter assemblies attached to the tool body at positions distributed azimuthally around the longitudinal axis of the tool, wherein each cutter assembly comprises a supporting structure bearing a plurality of cutters with exposed hard cutting surfaces providing the cutters' leading surfaces in the direction of rotation of the tool, and wherein the support structure is a body of particulate material infiltrated with a metal binder and incorporates a plurality of objects secured therein by the infiltrating binder.

One possibility is that these objects are the cutters with exposed hard surfaces, as already mentioned, which may have been partially embedded in the particulate material before the infiltration with binder metal.

Another possibility is that these objects are additional cutters, positioned circumferentially behind the cutters already mentioned, which may be embedded so that leading faces of these additional cutters are at least partially concealed within the body of particulate material but which can act as backup cutters in the event of damage to the first mentioned cutters.

A third possibility is that these objects are pieces of material having varying shape and size and having Knoop hardness of at least 1000 and possibly at least 1300 or at least 1600 which are partially embedded and partially exposed at a radially outward surface of the body of particulate material, so as to provide abrasive projections from it. These may be useful if the rotary tool is expandable, by assisting the cutting outward which allows expansion before the tool is advanced axially.

A fourth possibility is that these objects are pieces of material, providing smooth lubricious areas in a radially outward surface of the body of particulate material. This may reduce drag from sliding contact with a surface already traversed by one or more of the cutters. Material to provide lubricious areas may be bronze, copper or a high carbon steel.

Further aspects of the present disclosure are methods of making a rotary cutting tool by an additive manufacturing process comprising forming the body of particulate material which is the support structure as a succession of layers of particulate material, wherein at least some layers include particulate material having a Knoop hardness of at least 1000 and then infiltrating the particulate material with a molten metallic binder. The particulate material in the layers may change progressively in composition as some of the layers are deposited.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic, cross-sectional view of a drilling assembly in a borehole;

FIGS. 2 and 3 are cross-sectional elevation views of one embodiment of expandable reamer, showing its expandable cutter assemblies in retracted and expanded positions respectively;

FIGS. 4 and 5 are enlarged side and face views of a cutter assembly;

FIG. 6 is a schematic, cross-sectional view on line K-K of the outer block of

FIGS. 4 and 5 when the tool has been expanded in a pre-existing borehole;

FIG. 7 is a detail view of a PDC cutter;

FIGS. 8, 9 and 10 are cross sectional views through a mould, showing stages in making the outer block of FIGS. 4 to 6;

FIGS. 11, 12 and 13 are cross sectional views through the same mould, showing the effect of rotating it through an angle while filling;

FIGS. 14 and 15 are cross sectional views through different apparatus, showing stages in making the outer block of FIGS. 4 to 6 by another method;

FIG. 16 is a side view of a cutter assembly used in a different rotary tool;

FIG. 17 is a schematic, cross-sectional view on line K-K of the outer block, of FIG. 16;

FIG. 18 shows a possible modification;

FIG. 19 is a similar view to FIG. 9 showing a mould when used for making a tool with the modification of FIG. 18;

FIGS. 20 and 21 are cross sectional views through apparatus similar to that in FIGS. 14 and 15, showing stages in making an outer block by a hybrid method

FIG. 22 is a further view similar to FIG. 9 showing a mould when used for making a tool with another modification;

FIG. 23 is a further view similar to FIG. 9 showing a mould when used for making a tool with yet another modification;

FIG. 24 is an enlarged detail of the mould of FIG. 23; and

FIG. 25 is a cross section of a block made in the mould of FIG. 23.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary drilling assembly which includes an expandable under-reamer 22. A drill string 12 extends from a drilling rig 10 into a borehole. An upper part of the borehole has already been lined with casing and cemented as indicated at 14. The drill string 12 is connected to a bottomhole assembly 18 which includes a drill bit 20 and also includes an under-reamer 22 included in the drill string 12 at a position somewhat above the drill bit 20. The under reamer 22 is connected to the drill string 12 at standard connectors 40, 42 (indicated schematically) at upper and lower ends of the reamer.

The under reamer 22 has been expanded beneath the cased section 14. As the drill string 12 and bottomhole assembly 14 are rotated, the drill bit 20 extends a pilot hole 24 downwards while the reamer 22 simultaneously opens the pilot hole 24 to a larger diameter borehole 26.

The drilling rig is provided with a system 28 for pumping drilling fluid from a supply 30 down the drill string 12 to the reamer 22 and the drill bit 20. Some of this drilling fluid flows through passages in the reamer 22 and flows back up the annulus around the drill string 12 to the surface. The rest of the drilling fluid flows out through passages in the drill bit 20 and also flows back up the annulus around the drill string 12 to the surface.

As shown, the distance between the reamer 22 and the drill bit 20 at the foot of the bottom hole assembly is fixed so that the pilot hole 24 and the enlarged borehole 26 are extended downwardly simultaneously. It would be possible to use the same reamer 22 without the drill bit 20 in similar manner to enlarge an existing borehole.

Referring now to FIGS. 2 and 3, one embodiment of expandable reaming tool is shown in its retracted condition in FIG. 2 and in its expanded condition in FIG. 3.

This expandable tool comprises a generally cylindrical tool body 106 with a central flowbore 108 for drilling fluid. The tool body 106 includes the upper 40 and lower 42 connectors with tapered screwthreads for connecting the tool into a drill string 12. Intermediately between the connectors 40, 42 there are three recesses 116 formed in the body 106 and spaced apart at 120° intervals azimuthally around the axis of the tool.

Each recess 116 accommodates a cutter assembly 44 in its retracted position. The three cutter assemblies are similar in construction and dimensions. The radially outer face 45 of the cutter assembly 44 is indicated without detail in FIGS. 2 and 3. One cutter assembly is shown in larger side and face views in FIGS. 4 and 5. The side face shown by FIG. 4 is the leading face in the direction of rotation of the tool.

Each cutter assembly 44 comprises an inner block 46 and an outer block 48 which is a support structure for a number of cutters. The outer block 48 is bolted to the inner block 46 by bolts, not shown. Both side faces of the inner block 46 have protruding ribs 117 which extend at an angle to the tool axis. Ends 124 of ribs 117 are seen in FIG. 5. These ribs 117 engage in channels 118 at the sides of a recess 116 and this arrangement constrains motion of each cutter assembly such that when each cutter assembly 44 is pushed upwardly relative to the tool body 106, it moves bodily upwardly and radially outwardly from the position shown in FIG. 2 to an expanded position as shown in FIG. 3 in which the cutter assemblies 44 project outwardly from the tool body 106.

A spring 136 biases the cutter assemblies 44 downwards to the retracted position seen in FIG. 2. The biasing spring 136 is disposed within a spring cavity 138 and covered by a spring retainer 140 which is locked in position by an upper cap 142. A stop ring 144 is provided at the lower end of spring 136 to keep the spring in position.

Below the moveable cutter assemblies 44, a drive ring 146 is provided that includes one or more nozzles 148. An actuating piston 130 that forms a piston cavity 132 is attached to the drive ring 146. The piston 130 is able to move axially within the tool. An inner mandrel 150 is the innermost component within the tool, and it slidingly engages a lower retainer 170 at 172. The lower retainer 170 includes ports 174 that allow drilling fluid to flow from the flowbore 108 into the piston chamber 132 to actuate the piston 130.

The piston 130 sealingly engages the inner mandrel 150 at 152, and sealingly engages the body 106 at 134. A lower cap 180 provides a stop for the downward axial movement of piston 130. This cap 180 is threadedly connected to the body 106 and to the lower retainer 170 at 182, 184, respectively. There is sealing engagement between the lower cap 180 and the body 106.

A threaded connection is provided at 156 between the upper cap 142 and the inner mandrel 150 and at 158 between the upper cap 142 and body 106. The upper cap 142 sealingly engages the body 106 at 160, and sealingly engages the inner mandrel 150 at 162 and 164.

Expansion of the reamer is brought about by pressure of drilling fluid pumped down the drill string. The drilling fluid flows downwards in flowbore 108 along path 190, through ports 174 in the lower retainer 170 and along path 192 into the piston chamber 132. The differential pressure between the fluid in the flowbore 108 and the fluid in the borehole annulus surrounding tool causes the piston 130 to move axially upwardly from the position shown in FIG. 2 to the position shown in FIG. 3. A portion of the flow can pass through the piston chamber 132 and through nozzles 148 to the annulus as the cutter assemblies start to expand. As the piston 130 moves axially upwardly, it urges the drive ring 146 axially upwardly against the cutter assemblies 44. The drive ring pushes on all the cutter assemblies 44 simultaneously and moves them all axially upwardly in recesses 116 and also radially outwardly as the ribs 150 slide in the channels 118. The cutter assemblies 44 are thus driven upwardly and outwardly in unison towards the expanded position shown in FIG. 3.

The movement of the cutter assemblies 44 is eventually limited by contact with the spring retainer 140. When the spring 136 is fully compressed against the retainer 140, it acts as a stop and the cutter assemblies can travel no further. There is provision for adjustment of the maximum travel of the cutter assemblies 44. This adjustment is carried out at the surface before the tool is put into the borehole. The spring retainer 140 connects to the body 106 via a screwthread at 186. A wrench slot 188 is provided between the upper cap 142 and the spring retainer 140, which provides room for a wrench to be inserted to adjust the position of the screwthreaded spring retainer 140 in the body 106. This allows the maximum expanded diameter of the reamer to be set at the surface. The upper cap 142 is also a screwthreaded component and it is used to lock the spring retainer 140 once it has been positioned.

As seen in FIGS. 4 and 5, the outer block 48 of a cutter assembly 44 has an upper cutting region 51 provided with cutters 53 and a lower cutting region 55 provided with cutters 211-216. The upper and lower cutting regions 51, 55 are curved as shown by FIG. 4 so that the cutters in these regions are positioned radially outwards from the tool axis by amounts which are least at the top and bottom ends of the cutter assembly 44 and greatest adjacent the middle section which includes a stabilising pad 220 positioned to face and slide over the borehole wall.

The cutters 53 and 211-216 are polycrystalline diamond cutters (abbreviated to PDC cutters). As shown in FIG. 7 such cutters have a disc 57 of sintered diamond crystals formed at one end of a cylindrical body 59 which may be sintered tungsten carbide particles. The body 59 of each cutter fits into a cavity, often referred to as a pocket, formed in the cutter support structure which is the outer block 48 so that the cutter is partially embedded in the outer block 48 and its disc 57 of diamond particles is exposed as a hard cutting surface. Securing the body 59 of a cutter in a pocket in the outer block 48 may be done by brazing although it is also possible for a cutter to be secured mechanically in a way which allows it to rotate around its own axis thereby distributing wear. It has been normal practice for the hard disc 57 of diamond crystals to provide a flat cutting surface 58 as shown in the drawings. However, other shapes including cones can be used for the hard surface of a cutter.

In normal operation, the reamer is positioned in a borehole and then expanded as described above. Once expanded, the reamer is advanced downwardly within the borehole to enlarge the hole. The curved lower cutting regions 55 of its cutting assemblies 44 do the work of cutting through formation rock. This takes place in FIG. 1 as the drill string 12 is advanced downwardly. It is normal practice for most of the cutting by the reamer to be done as the reamer is advanced downwardly. However, the enlarged portion of the borehole can also be extended upwardly if required, using the upper cutting regions 51 to remove formation rock while pulling upwardly on the drill string 12. In the upper cutting region 51, the PDC cutters 53 are mounted so as to be partially embedded in the outer block 48 and project radially outwardly from the curved face of the block.

The leading side face of the outer block 48, seen in FIG. 4, has an area 204 which is slanted back as shown by FIG. 6. PDC cutters 211-216 are positioned in a row, in the slanted area 204 of the leading face of the outer block 48 with the hard surfaces of the cutters exposed. The cutters 211-215 are positioned at progressively increasing radial distances from the tool axis. The next cutter 216 is at the same radial distance from the tool axis as cutter 215.

The stabilising pad 220 has a part-cylindrical outward facing surface which is at the full gauge of the reamer and so when the cutter assemblies 44 are fully expanded, the outer surface of the stabilising pad 220 is part of a cylinder which is centred on the tool axis and lies on the notional surface swept out by the rotating tool. The outer extremities of the cutters 215 and 216 are also at the full gauge of the reamer and also lie on this notional surface. This notional surface is akin to a surface of revolution, because it is the surface swept out by a rotating body, but of course the reamer may be advancing axially as it rotates.

The outer surface 220 extends axially over the cutter 216 and over half of cutter 215. Thus, as shown by the cross-section in FIG. 6, the cutter 216 (and also cutter 215) has its extremity 218 aligned with outwardly facing surface area which is behind the leading faces of these cutters 215, 216 and follows these leading faces as the tool rotates.

The cutters 211-214 are partially embedded in the outer block 48 in a similar manner to the cutters 215, 216. The outer face of the block includes part-cylindrical surfaces 231-234 which extend behind the leading faces of cutters 211-214 respectively and which are aligned radially with the extremities of the respective cutters. Each of the part-cylindrical surfaces 231-234 has a radius which lies on the tool axis when the cutter assemblies 48 are fully expanded.

These surfaces 231-234 act as secondary gauge areas: the surface 231 slides over rock which has just been cut by the action of cutter 211, surface 232 slides over rock cut by cutter 232 and so on. These secondary gauge areas 231-234 contributes to stabilisation of the position of the rotating tool as it advances axially, even though the rock surfaces created by cutters 211-214 have only a transient existence, because they are cut away by cutters at a greater radius as the tool advances.

The outer face of the block 48 also includes curved portions 236 connecting the part cylindrical surfaces 231-234 and connecting the surface of the stabilising pad 220 to surface 234.

Referring to the cross section which is FIG. 6, the outer block 48 comprises a plurality of layers which differ in the composition of particulate material in them. Dashed lines indicate the interfaces between adjoining layers. Each layer is formed by particulate material and the particulate material of all layers is infiltrated with a metallic binder so that the outer block 48 becomes a single article.

The particulate material in the outer layer 252 is powdered tungsten carbide. This layer provides all the radially outward surfaces of the outer block, that is to say surfaces 231-234, 236 and 220 and also the outward surfaces in the upper cutting region 51. The particulate material in the inner layer 254 is steel. Within this inner layer 254, there is a channel 49 running along the length of the outer block. This fits onto a projection (not shown) from the inner block. In all the intermediate layers 256-259 the particulate material is a mixture of tungsten carbide and steel, in accordance with the following Table 1.

TABLE 1 Tungsten carbide Steel Outer layer 252 100%  Layer 256 80% 20% Layer 257 60% 40% Layer 258 40% 60% Layer 259 20% 80% Inner layer 254 100% 

Thus, the outer block 48 has a functionally graded composition with hard tungsten carbide at its radially outer surfaces, but steel which is more easily machined at its inner region. The progressive transition in composition reduces stresses arising from differences in the physical properties such as coefficients of expansion of the outer and inner layers.

The steel and tungsten carbide may differ in mean particle size and in their particle size distributions. Either or both may have mean particle size no larger than 100 micrometers, possibly no larger than 50 micrometers.

As can be seen, the cavity into which the cutter 216 is inserted is located in the outer layer 252. All the other cutters are at least partly in this outer layer 252 because the outer layer provides all the radially outward surfaces. However, it is possible that part of some cutters could project into the next layer 256.

There are several procedural routes by which the outer block 48 can be made. Some of them are adaptations of powder manufacturing procedures which are known for the manufacture of drill bits, for example procedures as disclosed in U.S. Pat. No. 6,353,771 or 7,832,457. It is possible to make the outer block 48 using a procedure in which a mould is first made by packing foundry sand around a pattern piece having the size and shape of the outer block 48. The pattern may be made by machining a convenient material, for example by CNC machining a block of aluminium. An alternative to using foundry sand is to make the mould directly by machining a block of graphite. Whether the mould is made by machining or by packing material around a pattern, the material of the mould should be a refractory material which is not wetted by the metallic binder used in the infiltration stage.

Another possibility is to make a mould by an additive manufacturing process in which computer-controlled equipment builds up the mould as a succession of layers one on top of another. The process includes selectively depositing material in each layer and/or selectively binding material in each layer, in accordance with a design stored in digital form. Such processes are known by various names including rapid prototyping, layered manufacturing, solid free-form fabrication and 3D printing. Apparatus for additive manufacturing is now commercially available from a number of manufacturers.

Some additive manufacturing processes deposit each layer as a uniform layer of separate solid particles extending across a working area and then selectively apply a binder material where required to form the desired object. Unbound particles are generally left in place until the object being made is complete but are then removed. The S-Print and S-Max systems available from ExOne, (of North Huntingdon, Pa., USA) implement a process of this type for making sand moulds and can be used for making a mould for an outer block 48.

FIGS. 8 to 10 illustrate manufacturing using a mould. FIG. 8 shows the cross section of a mould 260 for making the outer block 48 shown by FIGS. 4 to 6. This mould defines the radially outer surfaces and the side and end surfaces of the outer block. As seen in FIG. 8, surface 26 of the mould defines the radially outer face of the block 48, side surface 261 defines the leading face of the block and the other side surface 263 defines the trailing side face. After a mould has been made, whether it is made by additive manufacturing or by an older method of mould making, it is filled with the particulate material required for each of the layers. This may be done by hand, or by a machine dispensing the powder where required. The particulate material may be packed tightly in the mould by force and/or vibration.

The outer block 48 includes cavities for cutters and through holes for bolts which attach the outer block to the inner block 46. These internal spaces can be created by inserting sacrificial objects into the mould at the required positions and packing the particulate material around them. After infiltration, these sacrificial objects are destroyed to leave an empty space. Sacrificial objects may for example be made from graphite or from foundry sand. FIG. 9 shows the mould with particulate tungsten carbide for the outer layer 252 of block 48 placed in the mould and surrounding a graphite cylinder 264 which will subsequently form a cavity for a PDC cutter.

The particulate materials for layers 256-259 and lastly 254 are then placed in the mould. Transitions between adjacent layers are indicated by dashed lines in

FIG. 10.

FIGS. 11 to 13 show a variation on the procedure of FIGS. 8 to 10. The same mould 260 is used, but the mould is initially held at an angle as shown in FIG. 11 while the particular material for the layer 252 is placed in it. Consequently this layer 252 lies in contact with the side face 261 of the mould as well as face 262.

The angle of the mould is reduced progressively as particulate material for the subsequent layers 256-259 is placed in it, as shown by FIG. 12 for layers 256-258. The top of the mould is horizontal as seen in FIG. 13 when material for layer 254 is placed in it. The effect of filling the mould in this way is that the layer 252 with the greatest content of hard material provides much of the block's leading face defined by face 261 of the mould as well as the outer face of the block 48. The layers 256-259 vary in thickness across the block.

Both when the mould is kept level as in FIGS. 8-10 and when it is rotated as in FIGS. 11-13, after the mould has been filled, the particulate material in the mould is infiltrated with a molten metallic binder to unite the particulate material into a single solid article. This may be done in in a furnace and, if graphite sacrificial objects have been used, the furnace may be under vacuum or have an inert atmosphere so that the graphite is not ignited. The quantity of binder is chosen so that it will fill the interstices between the particles of material.

The binder may be drawn into the mass of particulate material by capilliary action and this may be assisted by gravity and by carrying out the process under vacuum, so that the infiltrating binder does not have to displace air. FIGS. 10 and 13 show the mould filled with particulate material, with a block of the metallic binder 266 placed on top ready to be heated in a vacuum furnace. When the binder melts, capillarity will draw the molten binder into the spaces between particles. Infiltration may also be assisted by applying pressure to the molten binder to force it into the particulate material. Pressure infiltration has been described by Cook and Werner in “Pressure Infiltration Casting of Metal Matrix Composites” Materials Science and Engineering A 144 pp 189-206 (1991).

The binder which is used for infiltration should have a melting point below the melting points of the particulate materials, so that the particulate materials remain solid while the molten binder fills the space between particles. The binder may be chosen to have a melting temperature of not greater than 1300° C. and possibly not greater than 1250, 1200 or even 1150° C. so as to have a melting temperature below the melting temperature range of iron and steel. The binder may be chosen to have a melting temperature of at least 800° C. and possibly at least 900 or 950° C.

The binder may be a single metal or a metal alloy. For example, copper has a melting point of 1084° C. and various compositions of brass melt in the range from 900 to 1000° C. U.S. Pat. No. 5,000,273, in the context of drill bit manufacture, discloses an infiltrating alloy of about 20 to 30% by weight of manganese, 10 to 25% by weight of zinc, and the balance copper. A specific infiltration alloy within these ranges is 20% by weight of manganese, 20% by weight of zinc, and the balance copper. This alloy composition is stated to have a melting point of about 835° C. thereby allowing infiltration to be carried out at about 950° C. WO2014/105595 discloses further alloys with a melting point of 815° C. (given as 1500° F.) or less.

The metallic binder may include elements which are non-metals. US2005/0211475 discloses infiltration binders which comprise at least one metal which is cobalt, nickel or iron, and at least one melting point reducing constituent which may be a transition metal carbide or may be boron, silicon, chromium or manganese. This document mentioned a number of specific compositions including (amongst others) an alloy of 45% tungsten carbide, 53% cobalt and 2% boron melting at 1151° C.;

an alloy of 45% tungsten carbide, 53% nickel and 2% boron melting at 1089° C., and an alloy of 96.3% nickel and 3.7% boron melting at 1100° C.

After the particulate material in a mould has been infiltrated to make it into a block, the layer 254 will have a flat surface at the level of the top of the mould 260. In a subsequent machining step this surface may be milled smooth, if that is required. The channel 49 along its length may also be made by machining. However, before these machining steps, the block may be removed from the mould and then heated in air so that sacrificial graphite cylinders burn away, leaving the bolt holes and cavities into which the PDC cutters are subsequently inserted. If foundry sand is used for sacrificial objects, it can be removed mechanically after the infiltration stage, for instance by drilling into it at low speed.

Another possible procedure for manufacturing is to use additive manufacturing to deposit the particulate material of the outer block 48 as a series of layers without a surrounding mould. There are several ways in which this can be done. One possibility is to deposit the particulate material of the outer block in a succession of cycles where each cycle has a first stage of depositing a layer of the particulate material uniformly across a working area and a second stage of selectively applying a liquid, non-metallic binder to bind particles in accordance with a digital design. A commercial process which uses this approach is the M-print system from ExOne.

A related, but slightly different approach, is to provide the required tungsten carbide and steel particles in the form of particles coated with a meltable organic coating. After each layer has been deposited, the required areas can be selectively heated to bind particles together and to the layer beneath in accordance with a digital design. A process of this type, using selective heating with a laser beam to fuse resin coated metal particles is disclosed in U.S. Pat. No. 5,433,280 for the purpose of making drill bits. In order to provide a functionally graded composition, the layers which are deposited may be progressively varied in composition, starting with tungsten carbide, then continuing using mixtures of tungsten carbide and steel and finishing with steel (or the other way around). Since the process uses a large number of thin layers it would be possible to have a very smooth transition from tungsten carbide to steel particles.

Once the cycles of layer deposition and binder application or cycles of layer deposition and laser melting of binder have been completed, any unbound material is removed as loose particles. The digital design for binding particles can define both the outer shape of the block and also cavities in it, including cavities for the insertion of cutters after infiltration.

An additive process which deposits the particulate material of an outer block 48 in this way provides a so-called green article in which the particulate material has the required composition and shape but the particles are bound together by an organic resin or some other non-metallic binder. Before infiltration, a mould of a refractory material is placed around the green article. If the shape of the green article permits, the mould can be made by packing a refractory material such as foundry sand around the exterior of the green article. If the green article includes cavities, it may be possible to pack foundry sand into them but an alternative is to make a mould around the green article using a flowable composition such as a cement slurry which can fill cavities and which then sets to a solid refractory mould. The mould and the green article within it are next heated in an oxidising atmosphere to burn away the non-metallic binder. The particulate material is then infiltrated with metallic binder as above.

A different approach to additive manufacturing is to deposit a plurality of particulate materials selectively to make both the block and a surrounding mould as a succession of layers. Apparatus for depositing the materials uses two or more devices which are controllable to deliver small quantities of powder and which are movable over a work area to deposit in accordance with a digital design.

For instance, one device delivers metal powder at required locations to form the block while a second device delivers a mould material at locations to form both the surrounding mould and any cavities within the block. After deposition has been completed, the article is infiltrated with metallic binder. The mould material is chosen so that it is not wetted by the molten binder and so remains in powder form which can be removed after infiltration has taken place.

A system for delivery of small amounts of dry powder was described by Yang and Evans in “A Multi-component Powder Dispensing System for Three Dimensional Functional Gradients” Materials Science & Engineering A: 379 pp: 351-359 (2004) and subsequently by Lu, Yang, Chen, and Evans in “Dry Powder Microfeeding System for Solid Freeform Fabrication,” 17th Solid Freeform Fabrication Symposium, Austin, Tex., pp. 636-643, 2006. Use of such a system to deposit particulate material surrounded by mould powder was described by Mohebi, Yang and Evans in “Computer generation of metal components by simultaneous deposition of mould, cores and part” 17th Solid Freeform Fabrication Symposium, Austin, Tex., pp: 490-501, 2006. This document mentions that the deposited particulate material was subsequently infiltrated with bronze.

Making the outer block 48 of FIGS. 4 to 6 in such a way is shown schematically in FIGS. 14 and 15. The apparatus resembles that shown in the Mohebi et al paper above. The apparatus has a refractory enclosure 270 surrounding a vertically movable table 272 which is raised and lowered by shaft 273 and supports a refractory top plate 274. Particulate material is deposited on the top plate of the table from three delivery nozzles 276 which are operable to deliver uniform small volumes of powder and are movable in two dimensions at a level very slightly above the top of the enclosure 270. One of these nozzles deposits steel particles, one deposits tungsten carbide particles and the third deposits a refractory material which is not wetted by the molten binder material and serves to form the mould. This powder may for instance be sand of small particle size or alumina powder.

The deposition of particulate material begins with the table 272 raised so that the upper surface of the plate 274 is at the same level as the top of the enclosure 270. After each layer has been deposited, the table is lowered within the enclosure 270 by the thickness of one layer and this continues until all layers have been deposited. The steel and tungsten carbide particles may be deposited in proportions which progressively change as the table is lowered, thus giving the block a functionally graded composition. The mould material is fine sand. FIG. 14 shows the positions of parts and materials after some of the layer 252 has been deposited. It can be seen that sand (which is the mould material) has also been deposited around the block at 278 and within a cavity 280 which will eventually receive cutter body 216.

FIG. 15 shows the positions when deposition is complete. The use of three deposition nozzles 276 makes it possible to deposit layers 256-259 which are not planar, even though the nozzles 276 move only two dimensions. As shown, the layer 252 of hardest material provides both the radially outer surface of the block and also the leading side surface. The layers 256-259 and the layer 254 are all exposed at the trailing side face of the block but contain a right angle turn part way across the block and do not reach all the way across to the leading face.

For the purpose of explanation this FIG. 15 shows the deposition of layers 256-259 with compositions as in Table 1 above. However, the use of nozzles which deposit thin layers makes it possible to provide a continuous gradual transition in composition from the tungsten carbide of layer 252 to the steel of layer 254.

Moulding sand filling the cavity 280 is continuous with the sand at 278 which provides a mould surrounding the deposited steel and tungsten carbide particles which will form the block. While depositing the layer 254, sand was also deposited at 282 to define the channel 49. Although this channel is formed during the deposition and subsequent infiltration, it may if required be milled to more exact dimensions after infiltration has taken place.

FIG. 15 shows that when the refractory enclosure 270 has been filled, the top plate 274 of the table comes to rest on an inwardly projecting lip 284, allowing the enclosure 270 and its contents to be moved to a vacuum furnace where infiltration with molten metallic binder is carried out.

FIGS. 16 and 17 show a rotary tool which is operated as a section mill used to remove a length of tubing, starting at a subterranean location which is some way down a borehole. An existing borehole is lined with tubing 72 (the wellbore casing) and cement 74 has been placed between the casing and the surrounding rock formation. The tubing and cement may have been in place for some years. The tool is useful to remove a length of tubing, starting at a point below ground. One possible circumstance in which this may be required is when a borehole is to be abandoned, and regulatory requirements necessitate removal of a length of tubing and surrounding cement in order to put a sealing plug in place.

The tool shown in these drawings is very similar to that shown in FIGS. 2

and 3 and uses the same operating mechanism, but the outer block 348 of each cutter assembly is different. FIG. 16 is a side view of outer parts of a cutter assembly when the tool is expanded and in operation. The side shown is the leading face in the direction of rotation. Numeral 306 indicates the outer wall of the tool body 106, exposed at the side of a recess 116 mentioned in the description of FIGS. 2 and 3. The inner block 46 is the same as in FIGS. 2 to 5. The outer block 348 incorporates cylindrical cavities which receive cutters 311 and 312 which are cylinders of compacted and sintered particles of tungsten carbide or other hard material. A further cutter 313 is cuboidal rather than cylindrical. Manufacturers of sintered tungsten carbide cutters include Cutting and Wear Resistant Developments Ltd, Sheffield, England and Hallamshire Hard Metal Products Ltd, Rotherham, England. The cutters are held in place by brazing. The outer block 348 comprises part-cylindrical outward facing surfaces which are aligned with the outer extremities of the cutter 311 and 312. This is illustrated by FIG. 17 showing the radially outer extremity 318 of cutter 312 positioned so that it is aligned with (i.e. is at the same distance from the tool axis as the part cylindrical surface 322.

For use as a section mill, the tool is attached to a drill string and lowered into the borehole tubing 72 to the required depth. The drill string is then rotated and the tool is expanded by pumping fluid into flowbore 108 as described above. The radially outer edge of cutter 313 contacts the interior face of the tubing 72 and cuts into it. This allows expansion to continue and the cutters 312 and 311 contact the inside face of the tubing 72 in sequence, cutting into and through the tubing until the fully expanded position of the inner and outer blocks 46, 348 is reached. The tool is then advanced axially in the direction indicted by arrow D. The leading cutter 311 on each cutter block is positioned to cut away any corrosion or deposits and also remove some material from the inside wall of the tubing 72, thus exposing a new inward facing surface 354. As the tool advances axially, the cutter 312 which extends outwardly beyond the cutter 311 removes a further thickness from the tubing 72, thus exposing a new inward facing surface 356. The remainder of the tubing indicated outside this new surface 356 is then removed by cutter 313 so that the full thickness of the tubing 72 has been removed. The cutter 313 also cuts into the cement 74 which was around the outside of the tubing. Radially outward facing surfaces of the outer block 348 thus make sliding contact with surfaces 354 and 356 transiently formed on the tubing 72 and with the cement 74.

Similarly to the outer block 48 in FIGS. 2 to 6, the outer block 348 is made of particulate material in layers which are infiltrated with metallic binder which to unites the layers of particulate material into a single article. As before, tungsten carbide is used for the radially outward layer 252 so that the surfaces which slide on the tubing 72 and the cement are hard and wear resistant. The particulate material in the inward layer 254 is steel and the intermediate layers 256-259 contain mixtures which give a progressive change in composition.

Manufacture can be as described for the outer block 48 of FIGS. 2 to 10. The outer block 348 may be made using sacrificial objects to create cavities for the hard cutters 311-313. However, it is alternatively possible that the cutters themselves are put in position in a mould and surrounded with particulate material before infiltration. The result is that the cutters are then secured in place by the infiltrating metallic binder. A further possibility is that sacrificial objects are used to create cavities, but these objects are removed and replaced with the hard cutters 311-313 before infiltration so that, again, the cutters are secured in place by the infiltrating metallic binder.

Securing tungsten carbide cutters with infiltrating metallic binder was tested experimentally by making a slab of infiltrated particulate steel with embedded cutters at one edge and then testing to failure by applying a load to a cutter in a direction which would also cause bending of the slab. The cutter cracked transversely to the applied load and the crack extended into the slab. The bond between the cutter and the surrounding slab remained intact, indicating a very strong bond.

FIG. 18 is a similar cross section to FIG. 17 but shows a modification. A second cutter 332 is embedded in the particulate material at a position where it follows the cutter 312 as the tool rotates. The cutter 332 serves as a back up. In the event that the leading cutter 312 becomes chipped or broken while the tool is operating, the infiltrated particulate material which follows the damaged cutter 312 may wear away even though it contains tungsten carbide particles. However, the embedded hard cutter 332 is at the same radial position as the damaged cutter 312 and the tool continues to function. Back up cutters could similarly be provided at positions following the cutters 311 and 313.

FIG. 19 is a cross section, akin to FIG. 9, showing a mould 260 being used to make the block of FIG. 18. The hard cutters 312 and 332 have been placed in the mould and surrounded with particulate material to make the radially outward layer 252. Subsequently the manufacturing process will continue as in FIGS. 8-10 with particulate material for layers 256-259 and 254 placed in the mould in succession, and the filled mould infiltrated with molten metallic binder which will also bind the cutters 312 and 332.

FIGS. 20 and 21 show process of manufacture which is a hybrid in that it makes use of both a preformed mould and also layered deposition of particulate material. The process is shown making a block with two tungsten carbide cutters as in FIG. 19 but the process is not limited to that and may be used for other forms of block.

The first part of the process is similar to that shown by FIGS. 8 to 10. A previously-formed mould 340 is somewhat shallower than the mould 260 shown in FIG. 8 but is otherwise similar. Cutters 312 and 332 are placed in the mould and the remaining space within the mould is filled with particular material for the outer layer 252 as shown by FIG. 20. The mould and its contents are then placed in apparatus very similar to that shown in FIGS. 14 and 15 with a refractory enclosure 270, plate 274 table 272 and shaft 273. The shaft, table and plate 274 are lowered sufficiently that the top of the filled mould 340 is aligned with the top of the enclosure 270. The mould 340 is dimensioned to be a close sliding fit within the closure 270.

The nozzles 276 are then used to deposit a succession of layers of material for the remainder of the block and also deliver mould material 344 around it. FIG. 21 shows the apparatus when deposition is complete. For purpose of explanation, layers 256-259 as in Table 1 above are shown although a gradual transition in composition is also possible. The layers 256-259 been deposited but are shaped so that they turn and do not extend all the way across to the leading side face of the block. This leading side face is formed by a column 342 of material which has the same composition as the particulate material of the layer 252. Thus the particulate material forming the leading and outer faces of the block has the highest concentration of hard material.

The enclosure 270 and its contents are then removed from the table 272 and transferred to a vacuum furnace for infiltration with molten metallic binder. The infiltrating binder infiltrates the particulate material deposited by the nozzles 276 and also the particulate material previously placed as layer 252 in the mould 320, thereby uniting all the particulate material into one solid mass and binding the sintered tungsten carbide cutters 312, 332 into place.

FIG. 22 shows a modification to the process for making the outer block of a reamer as in FIGS. 8 to 10. FIG. 22 is directly analogous to FIG. 9. The mould 260 contains a graphite cylinder 264 which will form a cavity for a cutter and contains the particulate material for layer 252. Additionally a bronze insert 347 has been placed in the mould 260 before the particulate material for layer 252. The manufacturing process will continue as shown by FIG. 10 with particulate material for layers 256-259 and 254 added to the mould, followed by infiltration with molten metallic binder. This binder will also bind the bronze piece 347. When cutters have been fitted into the cavities created by graphite cylinders such as 264 and the finished block is fitted to a reamer, the bronze insert 347 will provide a lubricious area at the outward facing surface of the block. Provision of a single lubricious area or multiple lubricious areas using multiple pieces of bronze or other lubricious material such as copper or high carbon steel could be done in this manner for either a reamer with PDC cutters or for a mill with sintered tungsten carbide cutters.

FIGS. 23 to 25 show a further possibility. FIG. 25 is a cross section through an outer block which is similar to the outer block 348 shown in FIGS. 16 and 17. The cross section is through the surface 323 axially behind the cutter 313 of FIG. 16. This surface is at the greatest radius of the outer block.

FIG. 23 is a cross section through the corresponding part of a mould for making this outer block. The first material placed in the mould 260 is pieces 370 of tungsten carbide made as a melt, cast and then crushed when cold. These have a Knoop hardness greater than 1600 and may well have a Knoop hardness over 1800. As best seen from the enlarged detail which is FIG. 24, these pieces 370 are mixed with sand 372 in an amount small enough that parts of the pieces 370 project into the mould interior from this sand 372. Next the particulate tungsten carbide for layer 252 of the outer block is placed in the mould as shown by FIG. 23. This is followed by the other layers of particulate material and all the particulate material is infiltrated with metallic binder as described with reference to FIGS. 8 to 10 The infiltrating metal binder does not wet the sand 372 but does adhere to the parts of the pieces 370 which project from the sand 372 into the mould interior. When the outer block is taken from the mould, its outward-facing surface has the pieces 370 partially embedded in it and partially projecting from it. Consequently this surface is rough and abrasive.

Giving the outward-facing surface a rough and abrasive texture by means of the projecting pieces 370 of hard cast tungsten carbide is useful during initial expansion of the tool. The projecting pieces 370 contribute to cutting outwardly through the tubing 72 as the tool is being expanded before it is advanced axially.

It will be appreciated that the embodiments and examples described in detail above can be modified and varied within the scope of the concepts which they exemplify. Proportions may be varied and in particular back raked cutting surfaces may be larger or smaller than shown in the drawings. Features referred to above or shown in individual embodiments above may be used together in any combination as well as those which have been shown and described specifically. More particularly, where features were mentioned above in combinations, details of a feature used in one combination may be used in another combination where the same feature is mentioned. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. 

1. A rotary cutting tool for incorporation into a drill string to enlarge a conduit underground, the rotary cutting tool comprising: a tool body having a longitudinal axis extending between upper and lower ends with connectors at the upper and lower ends for attaching to the drill string; and a plurality of cutter assemblies attached to the tool body at positions distributed azimuthally around the longitudinal axis; wherein: each cutter assembly includes a support structure bearing a plurality of cutters with exposed hard cutting surfaces providing the cutters' leading surfaces in a direction of rotation of the tool; the support structure has at least one surface positionable to contact the conduit wall; and the support structure comprises a body of particulate material infiltrated with a metal binder and the particulate material comprises material having a Knoop hardness of at least
 1000. 2. The rotary cutting tool of claim 1, wherein the particulate material has a composition that varies from a first region of the support structure bounded by its surface positionable to contact the conduit wall to a second region of the support structure attached to the body of the tool, and wherein the material having a Knoop hardness of at least 1000 is at a greater concentration in the first region than in the second region.
 3. The rotary cutting tool of claim 1, wherein the at least one surface positionable to contact the conduit wall is positionable to face radially outwardly and to align with a radial extremity of an exposed part of at least one cutter, so that the cutter does not project outwardly beyond the outward-facing surface and the outward-facing surface contacts the conduit wall after the cutter.
 4. The rotary cutting tool of claim 3, wherein the particulate material has a composition that varies from a radially outward region bounded by the outward-facing surface positionable to contact the conduit wall to a radially inward region attached to the tool body, and wherein the material having a Knoop hardness of at least 1000 is at a greater concentration in the radially outward region than in the radially inward region.
 5. The rotary cutting tool of claim 1, wherein the particulate material in a region of the support structure bounded by its surface positionable to contact the conduit wall comprises at least 50 wt % of particulate material having a Knoop hardness of at least
 1000. 6. The rotary cutting tool of claim 1, wherein the particulate material in a first region of the support structure bounded by its surface positionable to contact the conduit wall comprises at least 30 wt % of particulate material having a Knoop hardness of at least 1300, and wherein the particulate material in a second region of the support structure attached to the tool body comprises at least 75 wt % of particulate material having a Knoop hardness below
 800. 7. The rotary cutting tool according to claim 1, wherein at least 80 wt % of the particulate material has a particle size not exceeding 1 mm, and wherein the particulate material in a region of the support structure bounded by its surface positionable to contact the conduit wall comprises particles having a dimension of at least 5 mm and having a Knoop hardness of at least
 1300. 8. The rotary cutting tool of claim 1, wherein the particulate material in a region of the block bounded by its surface positionable to contact the conduit wall comprises: between 50 wt % and 95 wt % of particulate material having a Knoop hardness of at least 1000 and a particle size not greater than 1 mm, and between 5 wt % and 25 wt % of particles having a dimension of at least 5 mm and having a Knoop hardness of at least
 1300. 9. The rotary cutting tool of claim 1, wherein each cutter comprises a hard faced body of sintered particulate material having a Knoop hardness at an exposed hard face which is at least
 1500. 10. The rotary cutting tool of claim 1, wherein the cutters of each cutter assembly comprise leading cutters partially embedded within the support structure, with their leading faces exposed.
 11. The rotary cutting tool of claim 10, further comprising: additional cutters positioned rotationally behind the leading cutters and having radial extremities at the same radial distance from the tool axis as radial extremities of the leading cutters.
 12. A rotary cutting tool for enlarging an underground conduit, the rotary cutting tool comprising: a plurality of cutter assemblies attached to a structure of the tool at positions distributed azimuthally around a longitudinal axis of the tool; wherein: each cutter assembly includes a support structure bearing a plurality of cutters with exposed hard cutting surfaces providing the cutters' leading surfaces in a direction of rotation of the tool; the support structure is a body of particulate material infiltrated with a metal binder, the composition of the particulate material varying progressively from a first region of the support structure positionable to contact the conduit wall to a second region of the support structure spaced from the first region; and the first region contains particulate material with Knoop hardness of at least 1000 at a greater concentration than the second region.
 13. A rotary cutting tool for incorporation into a drill string to enlarge an underground conduit, the rotary cutting tool comprising: a tool body having a longitudinal axis extending between upper and lower ends with connectors at the upper and lower ends for attaching to the drill string; and a plurality of cutter assemblies attached to the tool body at positions distributed azimuthally around the longitudinal axis; wherein each cutter assembly comprises a support structure bearing a plurality of cutters with exposed hard cutting surfaces providing the cutters' leading surfaces in a direction of rotation of the tool; and wherein the support structure is a body of particulate material infiltrated with a metal binder and incorporates a plurality of objects secured therein by the infiltrating binder.
 14. The rotary cutting tool according to claim 13, wherein the plurality of objects are selected from the cutters, additional cutters positioned circumferentially behind the cutters, pieces of material having varying shape and size and having Knoop hardness greater than 1300 which are partially embedded and partially exposed at a radially outward surface of the body of particulate material, and pieces of material providing smooth lubricious areas in a radially outward surface of the body of particulate material.
 15. A method of making a rotary cutting tool for incorporation into a drill string to enlarge a conduit underground, the rotary cutting tool comprising a tool body having a longitudinal axis extending between upper and lower ends with connectors at the upper and lower ends for attaching to the drill string and further comprising a plurality of cutter assemblies attached to the tool body at positions distributed azimuthally around the longitudinal axis of the tool, the method comprising: making a support structure for each cutter assembly, the support structure bearing a plurality of cutters with exposed hard cutting surfaces providing the cutters' leading surfaces in a direction of rotation of the tool and the support structure having at least one surface positionable to contact the conduit wall, by depositing particulate material as a succession of layers of particulate material, wherein at least some layers include particulate material having a Knoop hardness of at least 1000; and infiltrating the particulate material with a molten metallic binder. 